Ray, I am not talking about equivalent prime lenses: I am not sure where you get that idea; the lenses that I have mentioned are a 12-50, 75-300, 12-100 and 100-400 zooms. My point is the simple fact (of which you are aware, as you have referred to it in other posts) that when a larger sensor with more pixels and pixel pitch 1.4x larger, getting the same pixel count on the same subject [the same angular resolution] require forming an image at the focal plane that is about 1.4x larger (linear), and so using a focal length 1.4x longer, and then cropping as needed (or using a longer focal length and so forming an even larger image, and then downsampling).
For example to match the telephoto reach of the long end of the Olympus 12-100mm on a 20MP MFT body like the EM1 Mk II would, on a D850, need at least 140mm, cropped to match FOV, and to match the long end of the Panasonic 100-400mm would require at least 560mm.
That said, your proposed matching of that 12-100 and 10-400 by a Nikon 24-120 plus [discontinued] Sigma 150-500 is close but falls a bit short: some combination like 24-140 and 200-560 would be needed. At the long end, one option I see is the Sigma 150-600/5-6.3 DG OS HSM | C.
Some weight comparisons:
Olympus 12-100/4 561 g
vs
Nikon 24-120/4 710 g
Just a bit heaver and "shorter" in reach.
Panasonic 100-400/4-6.3 985 g, $1800. C.f. 200-560
vs
Sigma 150-500/5-6.3 1780 g
A lot heaver and a bit shorter in reach.
or
Sigma 150-600/5-6.3 DG OS HSM | C 1930 g
Even heaver, with a bit more reach.
Okay! I get your point, BJL. In order to ensure that the D850 does not have a resolution disadvantage at the long end of the equivalent zoom, we need to use a longer focal lenth so that we need to crop the D850 image less, getting the same FOV whilst simultaneously achieving at least an equal pixel count of 20mp in both images.
That of course does increase the combined weight of the D850 system. I didn't know the Sigma 150-500 had been discontinued. If we use the Sigma 150-600 instead, the additional weight is only 150 grams. At 600mm, the D850 image, after cropping to the same FOV as the uncropped 400mm image from the EM1 MkII, would have more than 20mp, and each of those pixels is of better quality than the EM1 MkII pixel.
Lens quality issues are also a factor. It would be interesting to see a real-world comparison between the two lenses at the long end, on the different bodies. There might not be much difference at the long end but there sure will be at shorter focal lengths when the D850 doesn't need to be cropped at all, or might need to be be cropped only slightly to get the same FOV.
Adding up the weights of all these lenses and bodies, the 24-120 and 150-600 with the D850, and the 12-100 and 100-400 with the E-M1 MkII, the combined weights are; 2.12 Kgs for the MFT system, and 3.55 Kgs for the 35mm system.
That's an extra 1.43 Kgs for all the benefits of the much larger sensor with much higher pixel count most of the time when using zoom lenses, and better pixel quality.
As I understand, the main argument in favour of the Olympus 4/3rds system is that it provides sufficient image quality for your purposes, with an over all saving in weight and size, and therefore greater convenience of use. I can appreciate that.
If I were taking up digital photography as a new hobby, I would definitely consider opting for the Olympus 4/3rds system, now that they have a 20mp sensor with reasonably good quality pixels. However, in my current circumstances, already owning several Canon and Nikkor lenses, I would not be prepared to switch systems, unless the new system had no disadvantages, and only advantages.
P. S. Note that the EM1 Mk II has a pixel shifting high resolution mode matching about 40MP [Correction: about 50MP; I was thinking the EM5 Mk II], so if and when that can be used (tripod, good enough lens resolution, etc.) we are back fairly close to straight focal length equivalents, so needing a pair of F-mount lenses covering 24mm to about 800mm.
This is an interesting development. However, one way that I reduce the weight of my camera system nowadays, for the sake of convenience and the reduction of a lot of messing around, is not to bother with tripods anymore, unless they are really essential for a particular type of effect, such as the blurring of the water in a waterfall or taking shots at night, or in very poor light.
My impression is that pixel shifting requires a very stable tripod to avoid the slightest movement. Stable tripods tend to be heavy. That 1.43Kg of weight saving, which is the main justification for choosing the lighter MFT system, will be cancelled, will it not?
Also, pixel-shifting is only successful with static subjects, a bit like bracketing exposures for merging to HDR, so it's use is rather limited.
Have I made an irrefutable case?
P.S. Having done a bit of investigation on the pixel-shift feature of the E-M1 MkII, I'm getting the impression that the higher resolution is only significant when exceptionally good prime lenses are used. 90% of all lenses don't make the grade.